1. The Field of the Invention
The present invention relates to a hydraulic pump drive system for an automotive vehicle, and more specifically to a hydraulic pump drive system such that the hydraulic pump is driven by the engine starter motor in such a manner that when the engine is started, the starter motor is used as a series-wound motor to start the engine, and after the engine has been started, the starter motor is used as a shunt-wound motor to drive the hydraulic pump.
2. Description of the Prior Art
Conventionally, as a hydraulic pump driving system used for a power steering device for an automotive vehicle, there exists an hydraulic pump driving system driven by the engine through a belt or chain. In the prior-art hydraulic pump driving system, however, since performance capable of withstanding high speeds is required and additionally a sufficient quantity of hydraulic fluid is required even during idling, the hydraulic pump is inevitably large in size. Additionally, since the starter motor is only used for starting the engine and is idle except when the engine is started, the starter motor is not used efficiently.
On the other hand, it is well-known to have a series-wound starter motor for starting the engine, and a shunt motor for driving a hydraulic pump for power steering, power brakes, and the like in an automotive vehicle. The starter motor is employed solely to turn the engine during starting, which requires a large torque but occurs rarely. On the other hand, the auxiliary shunt motor need generate only a small torque but must continue to operate, at least intermittently, after the engine has been started. These disparate operational requirements have led to prior art systems which include two separate motors, even though the starter motor is used only to start the engine.